Deuteronomy 19:4-7

4And this is the case of the slayer who shall flee thither that he may live: he that smiteth his neighbour unwittingly, whom he hated not previously;

5as when he goeth into the wood with his neighbour to hew wood, and his hand fetcheth a stroke with the axe to cut down the tree, and the iron slippeth from the handle, and lighteth upon his neighbour, that he die; such an one shall flee unto one of these cities, and live;

6lest the avenger of blood pursue the manslayer, while his heart is hot, and overtake him, because the way is long, and smite him mortally; whereas he was not worthy of death, since he hated him not previously.

7Therefore I command thee, saying, Thou shalt separate three cities for thyself.